This invention relates to fuel cells in general and a method of managing the temperature level of a fuel cell in particular.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In practice, a number of these unit fuel cells are normally stacked or ganged together to form a fuel cell assembly. A number of individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. An alternate style of fuel cell has been recently proposed (U.S. Pat. No. 5,783,324) which is a side-by-side adjacent configuration in which a number of individual cells are placed next to each other in a planar arrangement. This is an elegant solution to the problems of gas transport and complexity of mechanical hardware.
The electrochemical reaction in a fuel cell is exothermic. The thus released heat must be removed from the fuel cell to avoid overheating of the latter. One of the ways of removing heat according to prior art is by using electrically conductive heat exchanger plates through which a cooling fluid flows. In this case, both sides of the membrane-electrode unit are in contact with the electrically conductive plate. Another prior art approach to cooling that is typical in a fuel cell stack is to use a dedicated coolant means in between individual fuel cells in stack. The dedicated coolant means is used to transport fluids through the fuel cell, which in turn removes the heat from the cells.
Fuel cell and stack designs having conventional cooling means such as the ones described above have several inherent disadvantages. First, conventional designs typically employ liquid cooling systems for regulating the cells"" operating temperature. Liquid cooling systems are disadvantageous because they require the incorporation of additional components to direct coolant into thermal contact with fuel cells. The power requirements to operate fluid handling components such as pumps and cooling fans, represent an additional parasitic load on the system, thereby decreasing the net power derivable from the stack. Such additional components also add volume, weight, complexity and cost to fuel cell designs.
Many hydrogen powered fuel cells use hydrogen supply from a hydride container as the fuel. Storage of hydrogen in a container containing reversible metal hydride is a common practice in the field of fuel cells. In order to release hydrogen from the hydride container heat has to be supplied to it, as the hydrogen release reaction is endothermic. If sufficient heat is not continuously supplied to the container during hydrogen release, the hydrogen flow from the container will cease. Also, the amount of hydrogen released has to be controlled such that it matches the target power output, pressure and concentration of hydrogen in the fuel cell system One method of providing heat to the hydride container is described in U.S. Pat. No. 4,826,741. The problem with methods of the prior art that transfer heat to the fuel storage container is that they use additional components such as separator plates to circulate coolants and active components such as pumps and valves to force the coolant through the system. Even the prior art approaches that try to minimize the use of active components by creating direct thermal conduction between the fuel cell and the fuel storage container are not effective as they use separator plates and depend on conducting heat to the edge of the separator plate. This causes non-uniform cooling of the fuel cell and the degrades the overall performance of the fuel cell system.
A typical fuel cell system for portable power application will have a fuel cell assembly (either of stack or planar design) and a hydrogen source in the form of a metal hydride container. During operation of a typical fuel cell system the fuel cell produces heat while the hydride container consumes heat. Therefore a suitable thermal management method that leverages the synergy between these two components is essential for efficient operation of the fuel cell system. It is also preferable to have a thermal management method that is passive and does not require additional components and that is self-regulating.
Although prior art techniques successfully regulate the temperature of a fuel cell and keep it within acceptable limits, they do so at the expense of overall fuel cell system performance. It would therefore be an advancement in the art of fuel cell systems to have a thermal management system that uses inherent thermal behavior and synergy within a fuel cell system to regulate its operating conditions and thus obviate the need for additional components such as separator plates, coolant plates, pumps, fans, valves, etc.